1. Field of the Invention
This invention relates to apparatus for optically inspecting an object having a reflecting surface to determine whether the object has any defect such as a flaw, stain and dirt.
2. Prior Art
Examples of objects to be optically inspected include a sheet 100 (FIG. 4), such as a polished metal sheet, having a front surface 100a serving as a mirror surface or reflecting surface, a transparent sheet 100 (FIG. 5) having a front surface 100a serving as a reflecting surface, and a multi-layer structure 100 (FIG. 6) composed of a plurality of layers, all of the layers being transparent except for the lowermost layer, and the uppermost layer having a front surface 100a serving as a reflecting surface. One example of the multi-layer structure is a semi-conductor board having a plurality of transparent coating layers thereon.
The sheet 100 of FIG. 4 is inspected to determine whether there is any defect a such as as flaw, stain or dirt on the reflecting surface 100a. The sheet 100 of FIG. 5 is inspected to determine whether there are any defects b on the reflecting surface 100a and the rear surface and in the interior. The multi-layer structure 100 of FIG. 6 is inspected to determine whether there are any defects c on the reflecting surface 100a and at the boundary between the layers and any separation d at the boundary between the layers.
FIG. 7 shows a conventional device for optically inspecting the objects 100 which device comprises a probe 110 having a light-receiving portion 111 at its front end, an irradiating means 112 in the form of an electric-light bulb placed in spaced relation to the probe 110, and a concave mirror 113 to which the irradiation means 112 is secured. Rays of light emitted from the irradiation means 112 are reflected by the concave mirror 113 and are directed toward an object 100 to be inspected, so that these rays of light are reflected by a reflecting surface 100a of the object 100 and are fed to the light-receiving portion 111 of the probe 110. Thus, the object 100 is illuminated with high brightness to enable any defect (for example, those defects indicated above by a, b, c and d) to be detected. However, this conventional device is relatively expensive since the irradiation means 112 and the light-receiving portion 111 are separate. In addition, time and labor are required for properly orienting the irradiation means 112 and the light-receiving portion 111. Further, this conventional device is rather space-consuming.
To overcome this difficulty, it has been proposed to provide a probe 120 (FIG. 8) which is similar in construction to a conventional endoscope and has a light-receiving portion 121 and an irradiating portion 122 at its front end. The front end of the probe 120 is disposed in opposed relation to the front surface 100a of the object 100 to irradiate it to carry out the inspection. With this probe however, an image of the irradiating portion 122 is reflected by the reflecting surface 100a of the object 100 and is fed to the light-receiving portion 121, thereby causing halation. This adversely affects the S-N ratio when processing an image signal obtained, and therefore an image of a defect present in the object can not be accurately detected.
It may be considered to dispose the probe 120 obliquely with respect to the object 100 to irradiate illuminating light to the reflecting surface 100a at an angle so as to prevent an image of the irradiating portion 122 from being fed to the light-receiving portion 121. In this case, however, the illuminating light is reflected by the reflecting surface 100a to irradiate the background, so that an image of the background is reflected by the reflecting surface 100a and is fed to the light-receiving portion 121. Therefore, in this case, the S-N ratio is also affected as described above, and an image of a defect present in the object can not be accurately detected.